The invention concerns an NMR magnet system for generating a highly homogeneous magnetic field of high field strength that is highly stable over time, with at least one superconducting magnet coil which is arranged in a first chamber of a cryostat in supercooled liquid helium at a temperature of less than 4.2K, with the cryostat having at least one further chamber containing liquid helium that is essentially at atmospheric pressure with a temperature of approximately 4.2K.
An NMR magnet system of this kind is known from a publication of Williams et al. in Rev. Sci. Instrum. 52 (5), May 1981, pp. 649-656 (American Institute of Physics).
Magnet systems for nuclear spin resonance devices are subject to extremely stringent demands in terms of achievable magnetic field strengths and their homogeneity. At a resonance frequency of 600 MHz, a field strength of 14.1 T (Tesla) must be achieved. The best technical approach to generating these high magnetic field strengths is by means of superconducting magnet coils that have a superconducting short-circuit switch. These superconducting magnet coils require energy only during the energization phase, and, after the current lead is removed, can generate a strong magnetic field without further energy input for a long period in short-circuit mode. In modern superconducting magnets, the time needed for field strength to decay to half its original value is on the order of 5,000 years. This means that in short-circuit mode over periods on the order of hours and days, essentially no change in magnetic field strength occurs. Excellent time stability is particularly necessary for long-term measurements, especially for so-called 2D and 3D measurements. These can be performed only in the superconducting short-circuit mode. In general, the magnet coils are energized once, and then generate a homogeneous magnetic field for years after the leads are removed. In routine operation, helium retention time for the magnet system is typically several months in the case of a "low-loss" cryostat.
NMR spectroscopy, however, requires not only high magnetic field strength but also extremely high spatial homogeneity of the magnetic field that is generated, since resolution is limited by the absolute homogeneity of the magnetic field. Line widths on the order of 10--10 are currently being achieved. At present, attaining even higher magnetic field strengths would require accepting compromises in terms of field homogeneity and field stability, meaning that higher signal strengths would need to be paid for with decreased resolution. This generally cannot be tolerated for NMR measurements. To date, field strengths of up to 20.7 T have been achieved with laboratory magnets in superconducting mode. However, these magnets can generate only low-homogeneity fields, and are generally unsuitable for NMR measurements.
The publication cited above proposes, as a way of obtaining stronger homogeneous magnetic fields and more stable superconductivity, that the superconducting magnet coil be operated at a lower operating temperature than the normal temperature of liquid helium (T=4.2K). This lower temperature is generally produced by pumping down the liquid helium.
The aforesaid document proposes a cryostat that has two concentric helium tanks nested within one another. The outer tank contains liquid helium at T=4.2K under standard pressure (1 bar). A filling line for liquid helium leads from this outer tank to the inner tank, so that liquid helium can be fed from the outer tank into the inner tank. In the inner tank, in which the superconducting coil is located, the helium is pumped down to a pressure of 40 mbar, thus cooling it to a temperature of 2.3K.
One major disadvantage of this arrangement is the fact that the supercooled helium in the inner tank is under partial vacuum, and that the electrical leads, especially those for energizing the superconducting magnet coil, must be passed through the cold vacuum system. This causes primarily sealing problems, but also insulation problems due to penetration of heat into the cold vacuum reservoir via the leads introduced from an environment at room temperature and standard pressure; these problems inevitably lead to greatly reduced helium retention times.